GREGARINTDA, BHIZOPODA, srONGIDA, AND INFUSORIA. 17 



leading structural features of Paramoecium is, in effect, a 

 definition of those of the group. 



Imagine a delicate, slipper-shaped body inclosed within a 

 structureless membrane, or cuticula, which is formed as an excre- 

 tion upon its outer surface. At one point (Fig. 4, B a) the body 

 exhibits a slight depression, leading into a sort of little funnel 

 (b c) coated by a continuation of the same cuticular investment, 

 which stops short at the bottom of the funnel. The whole of the 

 bag formed by the cuticula is lined by a soft layer of gelatinous 

 matter, or " sarcode," which is called the " cortical " layer 

 (Fig. 4, A a) ; while inside that, and passing into it quite gradu- 

 ally, there being no sharp line of demarcation between the two, 

 is a semi-fluid substance, which occupies the whole of the 

 central region of the body. Neither in the cuticle, the cortical 

 layer, nor the central substance, has any anatomist yet dis- 

 covered a differentiation into cellular layers, nor any trace of 

 that histological composition which we meet with in the tissues 

 of the higher animals ; so that here is another case of complex 

 vital phenomena proceeding from a substance which, in a histo- 

 logical sense, is structureless. 



At two points of the body (Fig. 4, A c c) the substance 

 of the cortical layer exhibits a remarkable power of contrac- 

 tion and dilatation. If you watch one of those points, the 

 sarcode suddenly seems to open like a window and, for a 

 while, a clear space is visible, which then, quite suddenly, shuts 

 again. After a little time the same diastole and systole are 

 repeated. As the systole takes place, it is possible, occasionally, 

 to discern certain radiating canals, which extend from the 

 cavities into the surrounding sarcode, and disappear again 

 before diastole occurs. There is no doubt that the clear space 

 is a chamber filled with fluid in the cortical layer, and since 

 good observers maintain that there is an aperture of communica- 

 tion, through the cuticula, between the ' contractile chamber ' 

 and the exterior, this fluid can be little more than water. Per- 

 haps the whole should be regarded as a respiratory or secretory 

 mechanism : in one shape or another, it is eminently charac- 

 teristic of the Infusoria. Besides this singular apparatus, there 

 lies embedded in another part of the cortical layer a solid mass, 



c 



